Good to hear you have a t&that your Mom is getting help too. Maybe you can find out from her more about her childhood or marriage that may still be affecting her. She may not want to talk about it, but that's OK-it would just add some insight. With the PTSD there are often things you don't remember at first-like the recall of your Mom doing it when you were younger. You can't trust the statement "I won't do it again" when you're dealing with someone with their own issues. I'm finally realizing that they really can't help it. My Mom used to yell&scream&beat me&I've really started to remember this to the detriment of good memories. If you can remember them, stick with the good ones if you can when this occurs-it can be helpful.
PS My Mom still chews me out or hangs up on me if I talk about my mental illness-it's as if it's the worst thing in the world to her&she's got it too! Thankfully she lives far away now.